In television, the image is formed by a number of horizontal lines arranged vertically down the display area. Each complete image frame is commonly produced as two interlaced fields, thus enabling a greater number of lines to be presented on the display without apparent flickering of the image than would be possible otherwise. Imaging systems producing video signals to this format perform the scanning of the image in a like manner, that is, as two interlaced fields.
However, an electronic imaging system is also often required to be used as an input sensor for electronic systems that may process or analyse the image and possibly produce some output actions as a result. In this case, the scanning of the image as a number of fields can cause difficulties: for example, if there is a moving object in the field of view then the time difference between scanning the object for each field causes a spatial displacement of the image of that object between the fields forming the complete image frame.
Accordingly, such systems ideally require the image scanning to be non-interlaced to minimise the time difference between the scanning of points in the image that are spatially close.
It is known to focus electromagnetic radiation from a scene onto a detector which converts this radiation into an electrical signal, the detector being subdivided vertically into a number of elements, such that each element produces a line in the output image. A continuous group of lines is scanned simultaneously and such a group of lines is termed a `swath`. The complete image is thus scanned as a number of such swathes adjacent vertically in the image (FIG. 2). The vertical scanning of these swathes is produced by relative displacement of the focussed image and the detector during the scan period. Such a displacement may be implemented, for example, by mechanical movement of a mirror in the optical path.
The vertical scanning rate depends on the vertical displacement required between swathes and the time taken to output the detector element signals for each swath. The detector element signal output time (swath period) will depend on how the image is scanned horizontally, for example, if a horizontal optical scanning mechanism is used, then the swath period will equal the horizontal scan repetition period of this mechanism.
Previously-known imaging systems provide either interlaced or non-interlaced scanning, but not both; it has been necessary to change mechanical or optical components of the system in order to change the scanning mode.